The Carter administration intends to stick with its goal of a 3 percent after-inflation increase in defense spending rather than acceding to new demands for higher amounts.

Several senators have suggested they will not vote for the pending strategic arms limitation treaty (SALT II) without a greater defense increase. But the administration has decided for now not to propose one, Pentagon aides said yesterday.

They noted that Defense Secretary Harold Brown said Friday on National Public Radio that a 3 percent increase for the next "several years," would "provide enough scope for us to maintain an adequate balance with the Soviets."

Brown's statement puts him and President Carter at odds with such influential senators as Sam Nunn (D-Ga.) and Henry M. Jackson (D-Wash.). They have called for a 4 to 5 percent annual increase, indicating this much extra spending would be necessary to make SALT II an acceptable risk.

But Carter, Brown and other administration executives have concluded it will be hard enough to get the majority of Congress to go along with a 3 percent increase, once the extra billions required are toted up, far less plump for 5 percent.

For openers, the Carter administration plans to ask Congress this fall for an extra $2.5 billion for the Pentagon's fiscal 1980 budget to make up for the amount eaten up by inflation not built into the original request.

The Senate Budget Committee, on the eve of Congress' August recess, shrank from raising the national defense ceiling high enough to provide the 3 percent real increase.

Also, the House defense appropriations subcommittee, after reviewing the Pentagon's request for fiscal 1980, cut the amount by $2.2 billion.

"I do not share that point of view" that holds that the SALT II treaty "can only be ratified if it is accompanied by a massive increase in federal defense spending," said Chairman Joseph P. Addabbo (D-N.Y.) in announcing his House subcommittee's action just before the August recess.

The Carter administration, after sorting thorugh conflicting advice from Congress and the Joint Chiefs of Staff, opted for a plan which calls for the 3 percent real increase between fiscal 1979 and fiscal 1980 as well as an unstated number of subsequent years.

Administration officials warn that while a 3 percent or 5 percent after-inflation increase may not sound like much, when the projected inflation rates are computed, the resulting numbers become so large that Congress is likely to balk at providing the money.

The Senate Budget Committee, in projecting what such increases would involve in terms of future dollars, figures that spending would have to jump from $130 billion in fiscal 1980 to $197 billion in 1984 to provide 3 percent real growth annually.

To achieve a 5 percent increase, the committee computed, the Pentagon budget would have to total $216.6 billion by 1984.

Since spending lags behind congressional appropriations, the totals that would have to be voted each year to achieve those percentage increases would be even larger.

To fund the national defense budgets Carter already has submitted for achieving a 3 percent increase, the committee estimated, would require Congress to approve $136.8 billion in fiscal 1980 and $189.4 billion in fiscal 1984.

but that was assuming a moderate inflation rate. Using higher inflation rates now projected by the Congressional Budget Office, the committee calculates that budget authority would have to jump from $141.2 billion in fiscal 1980 to $213.8 billion in fiscal 1984 to give the Pentagon a constant, 3 percent real growth.

To achieve 5 percent, the money voted for the Pentagon would have to start at $144 billion in fiscal 1980, according to the Senate Budget Committee, and keep rising every year, hitting $235.5 billion in fiscal 1984.

At the same time inflation is pushing up the total amount of money needed to provide the real growth in defense spending Carter has promised to NATO allies, the sophistication of modern weapons is giving the Pentagon a whole different set of financial problems.

These problems are behind the Defense Department's current drive to get the military to settle for simpler and cheaper weapons, with smaller aircraft carriers and attack submarines, two leading examples, now in contention.